10 Unpolished Mineral Specimens. Each rock or mineral specimen measures approximately 1-2 inches in length along its longest side, and can vary in color from grey to a dark pink. An igneous volcanic rock that is rich in silica, Rhyolite is known to have a texture that ranges from glassy all the way to porphyritic. This sample of Rhyolite is porphyritic, meaning that at least one group of crystals is distinctly bigger than others that are present. It is also extrusive, which is to say that the magma from which it was formed reached the surface of the Earth, either flowing out (extruding) to the surface, or exploding violently into the open air. It is considered to be the extrusive equivalent to granite, in part due to the resemblance that the can share. Rhyolite is related to obsidian, in that the difference lays primarily in how fast they were cooled. When cooled too quickly, the crystals form into a type of natural glass, known as obsidian. When it cools at a slower rate, microscopic crystals are allowed to form, allowing Rhyolite to occur. Origin: CO